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Old   August 21, 2002, 11:55
Default OUTPUT CONDITION
  #1
Memarzadegan,Amir Mohammad
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Dear all, I am rather new in cfd.i have writen SIMPLE and solve cavity.now i wish to extend my program to solve single input ,multiple outputs in an internal flow box(2D). when i assign pressure at output (which opens to atmospher (zero pressure)), i don't get good results(oscilatory ).

1.is it good to assign velocity at output? 2.is it necessary to assign pressure at outputs while i am assigning velocities? 3.how can i get good solution if i don't want to assign velocities at outputs?

I appreciate all your answers

A.M.Memarzadegan (M.S degree)Sharif University of Technology
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Old   August 21, 2002, 15:14
Default Re: OUTPUT CONDITION
  #2
Kevin A. Goodheart
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Dear A.M. Memarzadegan,

For an outlet boundary condtion there is 1 incoming characteristic which means you must ONLY specify 1 thing. When I solve a 2d nozzle I usually specify the pressure because that is what I know. If you know the pressure at the outlet then you have to specify the pressure, you must look elsewhere for the oscilatory behaviour. For example maybe the outlet boundary is too close. Another idea is to say that the average pressure at the outlet is equal to the pressure you specify, this will allow for some flexibility. The last idea is non-reflecting boundary conditions but I am still lost on that one so I can say much about it.

Kind Regards, Kevin
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Old   August 21, 2002, 21:03
Default Re: OUTPUT CONDITION
  #3
Farshid Bondar
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Hi Amir,

As a general CFD rule, under no circumstances we are supposed to over-specify boundary conditions. For instance, when you define velocity at the inlet, you should NOT specify velocity at the outlet. Regarding your oscillatory results, I recommend you to go through your code as Kevin suggested. Check out your pressure-correction equation once more.

Former Student of Sharif,

--Farshid
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Old   August 22, 2002, 05:35
Default Re: OUTPUT CONDITION
  #4
Memarzadegan,Amir Mohammad
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Thank you for your consideration,

Actually the outlets are not far enough to let the flow be fully developed and this is the main problem,because the sudden assignment of pressure cause divergence (i remind u i can get solution only for Re<50 (osilatory one)).

1.how can i prescirbed B.C in this situation(where outlets are close)?my professor says to fix velocity at outlets,in other word to assume we have specified suction fans at outlets.

I've checked my equations several times dear Kevin&farshid.

2.Is it true to say that if the code solves Cavity it has no problem? 3.how can i attach picture in this group?

Looking forward to hearing from you Memarzadegan,Amir Mohammad Sharif university
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Old   August 22, 2002, 08:44
Default Re: OUTPUT CONDITION
  #5
Kevin A. Goodheart
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Hello Amir,

O.K. so you have checked the equations so that either leaves the grid or the boundary conditions. Which brings us back to your question, So I suggest using the average pressure technique since you can not move the outlet farther down stream.

First calculate the average pressure on the outlet: P_avg = 0. sumdy=0. Do j=1,nj P_avg = P_avg+(P_j*abs(dy_j)) sumdy = sumdy+abs(dy_j) enddo P_avg=P_avg/sumdy

Now caculate the pressure on the outlet

Do j=1,nj P_j_outlet = P_exit/P_avg*P_(j) enddo

P_exit is the value that you specify.

I use this technique when I am solving an airfoil because the wake must pass through the outlet and I don't want to use a large grid (too much time) which would allow the wake to die out and come back to freestream conditions.

About specifying the velocity: it just seems strange because do you specify total velocity(but then how do you get the components) or just the u-component, do you assume the v-component is zero? Keep in mind though I am used to solving problems with nozzles where we apply a back pressure so that is why I am so used to using pressure.

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Old   August 22, 2002, 08:53
Default IGNORE THE FIRST ONE
  #6
Kevin A. Goodheart
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Now I know why they have the preview button.

O.K. so you have checked the equations so that either leaves the grid or the boundary conditions. Which brings us back to your question, So I suggest using the average pressure technique since you can not move the outlet farther down stream.

First calculate the average pressure on the outlet:

P_avg = 0.

sumdy=0.

Do j=1,nj

P_avg = P_avg+(P_j*abs(dy_j))

sumdy = sumdy+abs(dy_j)

enddo

P_avg=P_avg/sumdy

------------- Now caculate the pressure on the outlet ----------------------------------------

Do j=1,nj

P_j_outlet = P_exit/P_avg*P_(j)

enddo

P_exit is the value that you specify.

I use this technique when I am solving an airfoil because the wake must pass through the outlet and I don't want to use a large grid (too much time) which would allow the wake to die out and come back to freestream conditions.

About specifying the velocity: it just seems strange because do you specify total velocity(but then how do you get the components) or just the u-component, do you assume the v-component is zero? Keep in mind though I am used to solving problems with nozzles where we apply a back pressure so that is why I am so used to using pressure.

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